Color loss with Transparent Background
As it's been said, your setup lacks a source of white light. 

You can add such a source with a trick: an emissive and shadeless plane that is only visible through transparent objects, and completely transparent otherwise.


Add a plane big enough to fill your Camera view, place it in the background and parent it to the camera


Give it a material like the one in the figure below. The value in the Mix Shader to the left determines the maximum transparency that your objects will have in the rendered png (since this is a residual opacity of the plane itself). 

The only rays we are interested in are the ones pointed towards the camera, so uncheck all the other rays from the Object > Cycles settings panel


The result


You can unveil the plane's trick by looking at its boundaries